1. Field of the Invention
The present invention relates to a NOx concentration-measuring apparatus for measuring NOx contained in, for example, atmospheric air and exhaust gas discharged from vehicles or automobiles.
2. Description of the Related Art
Conventionally, those known as the method for measuring NOx in a measurement gas such as combustion gas include a technique in which the NOx-reducing ability of Rh is utilized to use a sensor comprising a Pt electrode and an Rh electrode formed on an oxygen ion-conductive solid electrolyte such as zirconia so that an electromotive force generated between both electrodes is measured.
However, the sensor as described above suffers the following problem. That is, the electromotive force is greatly changed depending on the change in concentration of oxygen contained in the combustion gas as the measurement gas. Moreover, the change in electromotive force is small with respect to the change in concentration of NOx. For this reason, the conventional sensor tends to suffer influence of noise.
Further, in order to bring out the NOx-reducing ability, it is indispensable to use a reducing gas such as CO. For this reason, the amount of produced CO is generally smaller than the amount of produced NOx under a lean fuel combustion condition in which a large amount of NOx is produced. Therefore, the conventional sensor has a drawback in that it is impossible to perform measurement for a combustion gas produced under such a combustion condition.
In order to solve the problems as described above, for example, Japanese Laid-Open Patent Publication No. 8-271476 discloses a NOx sensor comprising pumping electrodes having different NOx-decomposing abilities arranged in a first internal space which communicates with a measurement gas-existing space and in a second internal space which communicates with the first internal space, and a method for measuring the NOx concentration in which the O2 concentration is adjusted by using a first pumping cell arranged in the first internal space, and NO is decomposed by using a decomposing pumping cell arranged in the second internal space so that the NOx concentration is measured on the basis of a pumping current flowing through the decomposing pump.
Further, Japanese Laid-Open Patent Publication No. 9-113484 discloses a sensor element comprising an auxiliary pumping electrode arranged in a second internal space so that the oxygen concentration in the second internal space is controlled to be constant even when the oxygen concentration is suddenly changed.
A cermet electrode composed of Rh/ZrO2 is used for the NOx-decomposing electrode as described above. When the cermet electrode composed of Rh/ZrO2 is used for the NOx-decomposing electrode, a phenomenon has been observed, in which the sensitivity is lowered in accordance with the increase in operating time. This phenomenon is caused by the increase in impedance of the decomposing pumping cell. When the NOx sensor element, in which the impedance has been increased, is observed, it has been found that the contact area is decreased between the NOx-decomposing electrode and the ZrO2 substrate. In other words, it is postulated that the increase in impedance is caused by the decrease in contact area between the NOx-decomposing electrode and the ZrO2 substrate.
It is postulated that the decrease in contact area between the NOx-decomposing electrode and the ZrO2 substrate is caused by any change in volume which is brought about by the oxidation (Rh2O3) of the metal Rh contained in the NOx-decomposing electrode and the reconversion of the oxidized product into the metal.
The present invention has been made taking the foregoing problems into consideration, an object of which is to provide a NOx concentration-measuring apparatus which makes it possible to suppress the oxidation of Rh and the reconversion into the metal contained in a NOx-decomposing electrode, stabilize the impedance, and stabilize the measurement sensitivity.
The present invention lies in a NOx concentration-measuring apparatus for measuring a NOx concentration, comprising an oxygen pump and a NOx-decomposing electrode, the oxygen pump including an electrode having no decomposing/reducing ability for NOx or a low decomposing/reducing ability for NOx, to be used so that an oxygen concentration in a measurement gas is controlled to have a predetermined value at which NO is not substantially decomposable, and the NOx-decomposing electrode having a certain decomposing/reducing ability for NOx or a high decomposing/reducing ability for NOx, to be used so that NOx is decomposed to measure the NOx concentration by measuring an amount of oxygen produced during this process, wherein a cermet electrode composed of a Ptxe2x80x94Rh alloy and a ceramic component is used as the NOx-decomposing electrode.
The use of the cermet electrode composed of the Ptxe2x80x94Rh alloy and the ceramic component, as the NOx-decomposing electrode suppresses the oxidation of Rh and the reconversion of the oxidized product into the metal contained in the NOx-decomposing electrode. Even when the operating time is increased, it is possible to avoid the occurrence of the increase in impedance which would be otherwise caused by the decrease in contact area between the NOx-decomposing electrode and the substrate. That is, when the NOx concentration-measuring apparatus according to the present invention is used, the impedance is stabilized, and it is possible to stabilize the measurement sensitivity as well.
It is appropriate that a weight ratio of Rh in the Ptxe2x80x94Rh alloy contained in the NOx-decomposing electrode is more than 0% by weight and not more than 90% by weight.
Specifically, a ratio of Pt to Rh in the NOx-decomposing electrode is preferably Pt:Rh=10:90 to 99:1 as represented by a weight ratio. It is especially desirable that the weight ratio of Pt in the Ptxe2x80x94Rh alloy contained in the NOx-decomposing electrode is not less than 25% by weight, and Pt is not contained in an amount of 100% by weight. It is more preferable that the ratio of Pt to Rh in the NOx-decomposing electrode is Pt:Rh=25:75 to 75:25 as represented by a weight ratio.